A/C work then Not...?

[Andy4639]

Ok got 134-a system in the 71. Driving it to work everyday it will run fine. Blows out 40 degree air with passenger side vent open. The cowl vacum vent is gone I blocked it off.
Drive to NC 2hrs and about half way up it starts heating up. It's like it's freezing up. Noticed this morning coming home it was down to almost 30 degree's. Then it started heating up again.

[umwtnt]

Your probably correct. The expansion valve is probably freezing up due to moisture. If you have access to the proper tools, I would recover the refrigerant and pull a vacuum on the system for about 30 minutes and then recharge it. if that doesn't do the job then you will probably have to draw down the system, replace the dryer and pull another vaccum for about 15 minutes, then recharge. Sometimes it just easier to change the dryer upfront to save yourself time.

[Andy4639]

Did some checking. Pressures are good. It's getting good and cold and it's reading 28PSI. I don't think it's cycling on and off like it should and it's freezing it's self up. I ran it for almost 10 minutes in the garage and it never cycled. It just kept running.
Anybody know how to adjust the cycle switch on the acumalator?

[Andy4639]

I ordered a new switch. Will pick it up in the morning and see what it does.
Mine I have now has the screw in it to adjust. I hope the new one does to.

[Boog]

'.

[Boog]

I recently went through this on my 2000. It would take around an hour or so on the highway before I noticed it was warming up. Airflow through the vents dropped off as well.
New cycling switch did the trick on mine. I found one online for $27 but local parts store clipped me for about $45. I was robbed but figured the online purchase with shipping and handling would come in close to that plus I would have to wait for it.

[Fantom]

What year model dryer are you using...I noticed you're running a late model compressor also. Are you using the original condenser and evaporator?

Thanks,
Kevin

[Andy4639]

Originally Posted by Fantom

What year model dryer are you using...I noticed you're running a late model compressor also. Are you using the original condenser and evaporator?

Thanks,
Kevin

I swapped everything out several years ago from a 89 Chevy truck. No dryer just the acumalator now. Then when I swapped to 134a I bought the kit with brackets,hoses, expansion valve, compressor and condensor for 134a. The evaperator is the one out of the 89 truck. Didn't see a need to change it at the time. It's getting 40 degree's when it's 90 outside just on a long road trip it starts heating up. I hope it's just the cycling switch.

[mrein3]

I just thought of something...
Check your vacuum. You have a vacuum operated heater control valve and possibly vacuum to keep the door shut to keep the hot air out of the cowl.
If you have a vacuum leak and these controls aren't operating you may be getting too much of the ambient air and/or too much heat from the heater core through the system and into the cab.

[ls1nova71]

On the low pressure switch if you unplug the wiring connector there will be a small set screw, I forget which way to turn it, but one way will will make it cycle sooner and turning it the other will make it cycle later. I'd try adjusting it and see if it works better.
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[Andy4639]

Originally Posted by ls1nova71

On the low pressure switch if you unplug the wiring connector there will be a small set screw, I forget which way to turn it, but one way will will make it cycle sooner and turning it the other will make it cycle later. I'd try adjusting it and see if it works better.
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I tried all of that. That's why I think it's the switch it didn't matter where I had it it wouldn't cycle.

[Andy4639]

Well we ordered another switch this morning got home and the threads were wrong! I messed with mine some more and got it working again. I have no idea what I did other than played with the screw setting. It's cycling off about every 45 seconds now.

[partsman3]

Are you running an expansion valve or orifice tube? The 89 chevy had the oriface. You would normally use a accumulator, like you have, with an oriface and a filter drier with the expansion valve.

With the expansion valve system the compressor would run constantly with the valve regulating the refrigerant. With an oriface system the oriface size doesn't change, the pressure switch regulates the flow by cycling out the comp.

I cannot see your liquid line in the pics, so I can't tell if the oriface is there. On the 89 I believe it was behind the signal light, so you would have had to add one at the evap.

That said, your switch should correct the freezing up problem. Yes, some are adjustable but they need to be adjusted correctly. Cycling too often it will not cool at higher speeds and not cycling enough will cause a freeze up. Along with other problems.

Hope this helps somewhat.

[Andy4639]

Originally Posted by partsman3

Are you running an expansion valve or orifice tube? The 89 chevy had the oriface. You would normally use a accumulator, like you have, with an oriface and a filter drier with the expansion valve.

With the expansion valve system the compressor would run constantly with the valve regulating the refrigerant. With an oriface system the oriface size doesn't change, the pressure switch regulates the flow by cycling out the comp.

I cannot see your liquid line in the pics, so I can't tell if the oriface is there. On the 89 I believe it was behind the signal light, so you would have had to add one at the evap.

That said, your switch should correct the freezing up problem. Yes, some are adjustable but they need to be adjusted correctly. Cycling too often it will not cool at higher speeds and not cycling enough will cause a freeze up. Along with other problems.

Hope this helps somewhat.

Yes I have the oriface inline with the system. It's in the bottom line. I drove it a little ways about 10 miles round trip and it worked as it always did. It cycled on/off the whole time. I even let it idle as I dumped the trash and it stayed right at 40 degree. It was 91 by the temperature in the mirror. I guess next long trip will tell.

[partsman3]

Replace your pressure switch and see what happenes. Don't know how much you know about A/C systems. The 89 took R12. you said that you are using R134. If you are going by the label on the R12 system you may be over charged. When changing from R12 to R134 you only use 80% as much R134 as R12. Example 1 Lb is 16 oz, = 1lb of R12(16 oz) = 12.8 oz of R134. That could also cause your problem. The switch works off of pressure and R134 works at higher pressure than R12, thus not cycling making it freeze up.

There are still a lot of other things, but knowing what you are running (oriface) the switch is your most likely problem.

[Andy4639]

The AC has worked great for about 4 years now. I just have noticed this problem in the last couple of weeks. System is at right charge according to my gauges. pressures are all good.

I'm wondering if the screw moved on me some how over the past few years?
It seems to be running fine again now though. It maybe awhile before I go on another long trip but I'll keep a eye out on it.
Thanks for everones input on this.